Transcript Document

Four-Lane to Three-Lane
Conversions In Iowa:
A Summary
Results from Several Research
Studies of the Impacts on
Safety
Iowa Department of Transportation
Office of Traffic and Safety
Iowa State University
Center for Transportation Research and Education
Preface
The research results I am presenting
belong to others.
Their permission in doing so is greatly
appreciated.
The Researchers
Michael Pawlovich, Iowa DOT
Alicia Carriquiry, ISU Statistics Department
Win Lee, ISU Statistics Department
Tom Stout, CTRE
Keith Knapp, formerly CTRE
Karen Giese, formerly CTRE
Tom Welch, Iowa DOT (sponsor)
Presentation Outline
4 lane to 3 lane basics
Results from “traditional” safety studies
Results from Bayesian safety study
Conclusions
The Basics
• Converting an existing 4lane roadway to a 3lane roadway  typically
2-lanes with a center
continuous two-way leftturn lane (TWLTL)
• The addition of the turn
lane is the critical
ingredient for improved
traffic flow and safety
• May free up enough room
for bike lanes or parking
Source: Huang et al./HSIS 2002,2004,2005
The Iowa 4 Lane To 3 Lane
Experience
Before
After
Iowa DOT sees these conversions as positive in
terms of both safety and quality of life
Conversion Guidelines
When conversions seem to make sense



ADT < 20,000
Urban or small city arterial or collector with a
posted speed of 30 to 45 mph
History of left-turn related crashes
Broadside and rear end collisions

Significant commercial and/or residential
driveway density
Question: is there a density that is too high?

Significant left-turning volumes
Source: Iowa DOT RoadView
Two Example 4-Lanes
Clear Lake, IA – US 18 EB
(west of Clear Lake)
Storm Lake, IA – US 7 EB
Left-turning
vehicle slows
van

!!!
Source: Iowa DOT RoadView
Two Example 3-Lanes
Storm Lake, IA – US 7 EB
Storm Lake, IA – US 7 EB
Left-turning
vehicle has
no effect
Drive Through:
Sioux Center, IA
US 75 NB
1/8
Multiple
signs
Long transition length
Source: Iowa DOT RoadView
2/8
From
45 mph
Source: Iowa DOT RoadView
3/8
Passed after signage
Source: Iowa DOT RoadView
4/8
Plenty of time to change lanes
Source: Iowa DOT RoadView
5/8
Left-turning van
4-lane to 3-lane transition
Opposing flow queue headways
Source: Iowa DOT RoadView
6/8
Multiple left-turning pickups
Opposing flow queue headways
Parallel
Parking
Source: Iowa DOT RoadView
7/8
Signing and traffic markings
Source: Iowa DOT RoadView
8/8
3-lane to 4-lane transition
Source: Iowa DOT RoadView
Drive Through:
Sioux Center, IA
US 75 SB
From
45 mph
to
35 mph
Multiple
signs
Long
transition
length
Source: Iowa DOT RoadView
1/8
2/8
Multiple
signs
Source: Iowa DOT RoadView
3/8
3-lane section begins
Source: Iowa DOT RoadView
4/8
Parallel
Parking
Multiple left-turning vans,
pickup to turn right-on-red soon
Source: Iowa DOT RoadView
5/8
Parallel
Parking
Van finishes left movement,
pickup turns right-on-red
Source: Iowa DOT RoadView
6/8
$1.52/gallon!!!
Right-turning car,
van waiting for opposing left,
opposing thru traffic not hindered
Parallel
Parking
Source: Iowa DOT RoadView
7/8
Pedestrian signs
Parallel
Parking
More lefts and thru traffic
Source: Iowa DOT RoadView
8/8
3-lane to 4-lane transition
Source: Iowa DOT RoadView
Analysis History
 Treatments in Iowa (All Potential Case Studies)


15 from 1993 – 2003
2001: Guidelines (CTRE)
 4 to 3 Conversion Safety Impact Studies

2002: Huang et al. (FHWA/HSIS)
Showed small reduction in crash density and no reduction in
crash rate (this study is the “odd duck” here)


Fall 2004: Matched-Pair Before/After (CTRE)
Spring 2005: Bayesian Before/After (ISU Statistics
Department)
Yoked Pair Comparison
4-3 conversion segments were compared
with similar “control” segments
City population within 20%
Corridor traffic (AADT) within 20%
Physically similar (length, land use, access
management, etc.)
Yoked Pair Results
Net effect – 27 to 28% reduction in crash
rate
Crash Rate Analysis
Before – 8.86 per MVM
After – 4.45 per MVM
Older Driver Analysis
How did older drivers fare?
Age 65 and older – average crash rate of
7 before (on the 4 lane), 2.9 after (on the 3
lane)
Age 75 and older – average of 3.3 before,
1.5 after
Younger Driver Analysis
How about younger drivers?
Age 25 and younger – average of 28.1
before, 13.3 after
If you have teenagers, now you know why
their insurance rates are so high!
Injury Crashes Only
Net benefit, 27% reduction in injury
crashes
Conclusions
Overall crash frequency reduction of 21%
Improved crash rate (27-28% reduction)
Improvements for both older and younger
drivers
Reduced injury numbers and rate
Bayesian Before/After Study
 Objective: Assess whether 4 lane to 3 lane
conversions appear to result in crash reductions on
Iowa roads
 Control for “regression to the mean” phenomenon

Sometimes, crash rate improvements are simply random
 Much more rigorous study than the “traditional” study
just described
Bayesian Before/After
 Data

15 treatment and 15* control

23 years (1982-2004)

Volumes: 2,030 to 15,350 (1982-2004)
* One control site served as a matched pair for two treatment sites.
Data  3-lane (treatment sites)
SID
CITY
LITERAL
CIPOP
ADT
2000
2000
1
Storm Lake
Iowa 7 from Lake Ave. to Lakeshore Dr.
10,076
7,503
2
Clear Lake
US 18 from N 16 St. W to N 8th St.
8,161
10,403
3
Mason City
Iowa 122 from West intersection of Birch Drive to a Driveway
29,172
7,800
4
Osceola
US 34 from Corporate limits on east side to where highway divides to 4 lanes on west side
4,659
8,172
5
Manchester
Iowa 13 from River St. to Butler St.
5,257
9,400
6
Iowa Falls
US 65 from City Limits - ? to Park Ave.
5,193
10,609
7
Rock Rapids
Iowa 9 from S Greene St. to Tama St.
2,573
4,766
8
Glenwood
US 275 from MP 36.2 to MP 37.42
5,358
6,410
9
Des Moines
Beaver Ave from Urbandale Ave. to Aurora Ave.
198,682
13,695
10
Council Bluffs
US 6 from McKenzie Ave. west 1300 ft.
58,268
11,000
11
Blue Grass
Old US 61 from Oak Lane to 400' W of Terrace Drive
1,169
9,155
12
Sioux Center
US 75 from 200' South of 10th St. S. to 250' North of 9th St. NW
6,002
8,942
13
Indianola
Iowa 92 from South R St. to Jct. of US 65/69
12,998
13,288
14
Lawton
US 20 from 100' east of Co. Rd. Eastland Ave. to 1130' West of Co. Rd. Emmet Ave.
697
9,237
15
Sioux City
Transit Ave. from Vine Ave. to just west of Paxton St. at curve
85,013
9,608
Data  4-lane (control sites)
SID
CITY
LITERAL
CIPOP
ADT
2000
2000
18
Storm Lake
Iowa 7 from Lake Ave. to Barton St
10,076
8,790
19
Le Mars
US 75 from 0.01 miles north of 3rd St NW to 0.36 miles SW of 12th St SW
9,237
10,880
20
Cedar Falls
Green Hill Road from 0.10 miles east of IA 58 to 0.09 miles west of Cedar Heights Dr.
36,145
2,768
21
Jefferson
Iowa 4 from National Ave to 0.13 miles north of 250th Ave
4,626
5,685
22
Harlan
Iowa 44 from US 59 to 6th St
5,282
6,981
23
Norwalk
Iowa 28 from 0.03 miles south of Gordon Ave to 0.04 miles south of North Ave
6,884
7,679
24
Belmond
US 69 from 0.38 miles north of Main St to 0.58 miles south of Main St
2,560
3,734
25
Harlan
Iowa 44 from US 59 to 6th St
5,282
6,981
26
Des Moines
Hickman Road - 40th Place east to 0.07 miles west of W 18th St
198,682
13,953
27
Ames
13th Street from 0.09 miles east of Stange Road to 0.07 miles west of Crescent Circle Dr.
50,731
10,711
28
Mapleton
Iowa 141 from 0.02 miles north of Sioux St. to 0.08 miles south of Oak St.
1,322
3,007
29
Algona
US 169 from 0.07 miles south of US 18 to 0.23 miles south of Irvington Rd.
5,741
7,263
30
Oskaloosa
Iowa 92 from 0.12 miles east of IA 432 to 0.07 miles west of Hillcrest Dr
10,938
11,143
31
Merrill
US 75 from 0.05 miles north of 2nd St to 0.18 miles north of Jackson St
754
7,774
32
Sioux City
S. Lakeport from 4th Ave to Lincoln Way
85,013
15,333
Data  Pairings*
Treatment
Control
SID
ROUTE
LENGTH
COMPYEAR
SID
ROUTE
LENGTH
1
IA 7
1.41
1993
18
IA 7
0.71
2
US 18
1.51
2003
19
US 75
1.80
3
IA 122
1.78
2001
20
Local
1.80
4
US 34
2.04
2001
21
IA 4
2.40
5
IA 13
0.35
2001
22
IA 44
1.20
6
US 65
1.23
2002
23
IA 28
0.80
7
IA 9
0.35
1998
24
US 69
0.90
8
US 275
1.09
1998
25
IA 44
1.20
9
Local
1.19
1999
26
Local
1.50
10
US 6
0.20
2000
27
Local
0.33
11
Local
0.72
1999
28
IA 141
0.70
12
US 75
1.52
1999
29
US 169
2.00
13
IA 92
1.57
1999
30
IA 92
1.50
14
US 20
0.64
2000
31
US 75
0.50
15
Local
0.77
2000
32
Local
1.20
* Not essential for Bayesian, more for discussion purposes
Monthly Crash Densities
Treatment
Control
Monthly Crash Rates
Modelling/Statistics
 Crashes: random, rare count events
 Model: Poisson w/ log mean expressed as a piecewise linear function of:
- time period (months)
- seasonal effects
- random effect for each site
 Adopted a Bayesian statistical framework for
estimation of model parameters
Results/Discussion
 25.2% (23.2% - 27.8%)
in crashes/mile
25%
 18.8% (17.9% - 20.0%)
19%
in crash rate
Results/Discussion
 Agrees with:


CTRE Matched-pair Before/After study
Why? Prior Sioux Center speed study
 4-5 mph in 85th percentile free flow
speed
 12-14% in percentage of vehicles
traveling more than 5 mph over the speed limit
(i.e., vehicles traveling 35 mph or higher).
Conclusions
4 lane to 3 lane conversions appear to be
successful treatments for calming traffic
under the right conditions
They can be very effective in improving
traffic safety
They are not especially problematic for
either elderly or young drivers